Monopolar dc to bipolar to ac converter

ABSTRACT

This invention improves the performance and lowers the cost of DC to AC inverters and the systems where these inverters are used. The performance enhancements are most valuable in renewable and distributed energy applications where high power conversion efficiencies are critical. The invention allows a variety of DC sources to provide power thru the inverter to the utility grid without a transformer and at very high power conversion efficiencies. The enabling technology is a novel inverter circuit topology where the DC source is connected directly to the positive bus of the DC to AC converter and where a negative bus is generated from the positive bus with a flyback converter. The inverter power topology does include or require a transformer. The AC inverter output configuration can be either single-phase or three-phase. The AC inverter output can be either utility interactive or directly supply loads.

DETAILED DESCRIPTION

[0001] The invention is more related to the power circuit topology of an inverter than the control methods. The inverter topology is novel while the control methods are known. The preferred embodiment of the invention is shown in FIG. 1 and is illustrated as a system consisting of three components; an inverter 30, a photovoltaic array 20 and a typical 60 cycle 120 Vac, residential, electric utility service 40. The system converts solar energy to electric power and functions as a distributed generator on the electric utility grid. Photovoltaic cells produce power over a wide voltage range depending on the amount of sunlight, ambient temperature and wind speed. Additionally, there are National Electric Code and class-of-equipment restrictions that make photovoltaic arrays more cost effective when sized for a maximum of 600 Vdc. To source AC power into the electric utility grid, over the expected range of DC voltages, prior art utility-interactive inverters would either need an external 60 cycle distribution transformer or a dual boost converter input stage to achieve the proper voltage match. A distribution transformer decreases power conversion efficiency and adds to the overall system costs while a dual boost converter adds complexity to and lowers the conversion efficiency of the inverter. The invention limits the number of conversions required from two, either the dual boost conversion and the DC to AC conversion or the DC to AC conversion and the 60 cycle step-up transformer conversion in the prior art, to 1½ conversions with said invention. This is because the DC to AC converter only processes ½ of the throughput power, the positive monopole, once. The negative monopole is processed twice as in the prior art. The prior art, dual boost converter is old technology and is well known. In FIG. 1, a monopolar photovoltaic array 1 is grounded with the return terminal connected to grounding electrode 2. The photovoltaic array 1 is connected across energy storage capacitor 8. Capacitor 8, Insulated Gate Bipolar Transistor (IGBT) 5, inductor 6, diode 7 and capacitor 9 are configured as a typical, transformerless flyback converter. This circuit configuration is known. The output of current sensor 4 is used in conjunction with the voltage sensed across capacitor 9 to regulate the voltage across capacitor 9. The voltage on capacitor 9 is regulated to slightly higher absolute value than the negative peak voltage on the utility grid 17. With a properly sized photovoltaic array 1, there is positive bus or monopole across capacitor Sand a negative bus or monopole across capacitor 9 both with respect to ground. The DC to AC converter is a known half-bridge configuration with IGBT switches 10 and 12 and freewheeling diodes 11 and 13. The Pulse Width Modulated (PWM) sinusoidal current regulation method for utility interactive inverters is known. Inductor 14 and capacitor 16 form a 2-pole filter that removes high frequency PWM components. The control circuit uses current sensor 20 to regulate sinusoidal current into the utility grid, synchronized with the utility grid voltage for unity power factor power transfer. The control circuit also uses current sensor 20 to precisely regulate DC current components to near zero. These control algorithms are known. The switching frequency is typically upwards of 20 kilohertz. This invention facilitates high power, high frequency, lower cost DC to AC power conversion over a wide DC input range with a minimum number of conversions. 

1. An inverter for utility interactive applications for converting DC power from solar photovoltaic modules into AC power and comprising: a DC to DC flyback converter that generates a negative voltage with respect to ground. a DC to AC converter which converts a DC input voltage and the output from said flyback converter into current regulated sine waves, synchronized with the utility voltage and sourcing power into the utility grid.
 2. A system for generating AC power from DC sources comprising: a DC source; a DC to DC flyback converter that generates a negative voltage with respect to ground. a DC to AC converter which converts a DC input voltage and the output from said flyback converter into current regulated sine waves, synchronized with the utility voltage and sourcing power into the utility grid.
 3. An inverter according to claim 1 wherein the switching devices are insulated gate bipolar transistors.
 4. An inverter according to claim 1 wherein the switching devices are field effect transistors.
 5. An inverter according to claim 1 wherein the switching devices are a combination of insulated gate bipolar transistors and field effect transistors.
 6. An inverter according to claim 1 wherein the DC to AC converter section is configured for split-phase operation using two half-bridge circuits.
 7. An inverter according to claim 1 wherein the DC to AC converter section is configured for single-phase operation using one half-bridge circuit.
 8. An inverter according to claim 1 wherein the DC to AC converter section is configured for three-phase operation using three half-bridge.
 9. A system according to claim 2 wherein the DC source is grounded.
 10. A system according to claim 2 wherein the DC source is not grounded.
 11. An inverter according to claim 1 wherein the intended DC source is a fuel cell, battery, genset, wind turbine or micro-turbine.
 12. An system according to claim 2 wherein the DC source is a fuel cell, battery, genset, wind turbine or micro-turbine.
 13. An inverter according to claim 1 wherein the output supports loads directly and does not necessarily have utility interactive capabilities.
 14. A system according to claim 2 wherein the inverter output supports loads directly and does not necessarily have utility interactive capabilities. 